Optical fibre strain sensors are known and WO 2004/056017 discloses a method of interrogating multiple fibre Bragg grating strain sensors along a single fibre. In the system of WO 2004/056017, Bragg gratings are defined in the optical fibre at spaced locations along the optical fibre. When the optical fibre is put under strain, the relative spacing of the planes of each Bragg grating changes and thus the resonant optical wavelength of the grating changes. By determining the resonant wavelength of each grating, a strain measurement can be derived for the location of each grating along the fibre. The light reflected from each fibre Bragg grating is identified by the time of arrival of a reflected light pulse at a detector, such that the reflected signals from multiple gratings in a single fibre are multiplexed in the time domain.
A problem with existing time-division multiplexing (TDM) devices is that due to the operating speeds of the electronics in the TDM instrument, the sensors, or fibre Bragg gratings, must be placed at relatively large distances apart so that the electronics is not bombarded with multiple return signals at any one moment. Typically, the electronics cannot cope with signals returning from spacings between sensors of less than 1 meter as the signals arrive too quickly for the electronics to analyse one signal before the next arrives.
One solution to this problem is to use wavelength division multiplexing (WDM). In this case, each grating in a single optical fibre has a resonant wavelength in a different discrete wavelength band. In this way, the reflected light from each grating can be identified by the resonant wavelength of light reflected, which means there is no limitation on the location of the gratings along the fibre. However, for such a system to function effectively every grating in a single fibre must operate in a discrete wavelength band and this imposes limitations on the construction of a sensor system.
The present invention, at least in its preferred embodiment, seeks to obviate one or more of the disadvantages of known interrogation devices.